1. Field of the Invention
The present invention relates generally to instrumentation devices used to transmit level indication signals from a remote location to a local indicating station or microprocessor and, more particularly, to a remote optical sensor head connected to a laser measurement device via fiber optic cables that is used to monitor levels in liquid storage vessels.
2. Description of the Related Art
In many industrial applications, it is common practice to monitor fluid levels in liquid storage vessels located in remote areas using instrumentation equipment that transmits a level indicating signal to a centralized location for monitoring and control purposes. In the past, several methods have been utilized in obtaining these remotely generated level indicating signals. However, most are flawed in the sense that they are susceptible to drawbacks and intrinsic characteristics that often result in inaccurate measurements.
Mechanical, float-type level indicating devices have been used to indicate fluid levels by placing a float in the vessel itself. As the float raises and lowers with the fluid level in the vessel, a variable resistance electronic device is manipulated, thus providing a means by which the level is measured in proportion to the variable resistance. While this method provides accurate readings, it requires frequent calibration and is limited in use to liquids that will not deteriorate the hardware. Furthermore, turbulent conditions within the vessel can also lead to inaccurate measurements. Similarly, mechanical float-type level limit switches suffer from the same limitations as the level indicators, although calibration is not as great a factor.
The drawbacks associated with the aforementioned devices gave rise to the development of other devices that do not rely on direct contact with the liquid. Ultrasonic level detectors utilize a transmitter/receiver to detect liquid levels by transmitting a signal from the top of the vessel and monitoring the time required for it to be reflected off the surface of the liquid and return to the receiver. While this method eliminates many of the problems associated with mechanical devices, it too suffers from inaccuracies caused by varying temperatures and densities in the area above the fluid level inside the vessel.
Using principles similar to those of ultrasonic indicators, microwave and radar level indicators use their respective signal types to indicate liquid levels without coming into direct contact with the liquid. However, these methods too suffer from limitations in the sense that liquids with poor dielectric constants cause inaccurate level indications.
Other limitations associated with the ultrasonic, radar and microwave devices results from the fact that these devices are electronic in nature and therefore suffer from intrinsic safety concerns. As with any electronic device, there are, necessarily, heat generating components that create, no matter how remote, the possibility of hazardous situations when used in areas where flammable materials are present. Furthermore, electronic devices also suffer from complications created by line noise, electromagnetic field interference (EMF) and grounding problems. The aforementioned limitations associated with the use of these devices has lead to the development of alternative methods, including the use of laser and optical instrumentation devices. However, depending upon their configuration, these devices also suffer from inadequacies and drawbacks that identify the present invention as being a superior method of measurement
In the ancillary art, several devices have been developed that utilize laser based devices to calculate and indicate fluid levels within liquid storage vessels. For example, U.S. Pat. No. 5,257,090, issued in the name of Meinzer et al, discloses a laser liquid level/distance measuring device wherein a laser beam is aimed vertically downward into the vessel and is reflected from a floating reflector back to an optical receiver. The level is then calculated as being proportional to the time required for a transmitted signal to be reflected back to the receiver. This method suffers from drawbacks because, as previously mentioned, the device requires submersion in the liquid itself which can lead to problems associated with corrosion and turbulence within the vessel. This method also suffers from the aforementioned intrinsic safety concerns related to electronic devices.
U.S. Pat. No. 4,938,590, issued in the name of Ishida, similar in nature to the Meinzer invention, includes a means to compensate for varying vessel pressure. However, this invention also suffers from the corrosion and turbulence problems because it also requires submersion and is susceptible to intrinsic safety concerns due to its electronic nature.
U.S. Pat. No. 5,020,901, issued in the name of de Groot, discloses a laser range detector used to measure the distance between objects. Although related to the present invention in the sense that it uses a similar method for measurement, this device is not intended for use in level measurement. Furthermore, this device relies on a reflector being located at the same location as the remote object. Therefore, any adaptation for liquid level measurement would require submersion and result in the same problems associated therewith.
U.S. Pat. No. 5,278,426, issued in the name of Barbier, discloses an optical liquid level limit switch-type sensor wherein the sensor module is mounted on the sidewall of the vessel facing the interior thereof. The device consists of a light transmitter and a light detecting receiver. In the absence of liquid at the sensor level, the amount of light reflected and detected by the receiver is substantially greater than when the liquid is present. Thus the device indicates when the liquid in the storage vessel reaches a level equal to that of the sensor. While this invention overcomes the problems associated with submersion previously discussed, it does rely on electronics being located at the sensor and therefore suffers from the problems associated therewith, including noise, EMF, grounding and fire hazards.
U.S. Pat. No. 3,995,169, issued in the name of Oddon and U.S. Pat. No. 4,051,726, issued in the name of Hastbacka, disclose optical level indicating devices for liquid storage vessels wherein transparent, light conducting probe rods are submersed in the liquid. Knowing that the amount of light transmitted through the probe is a function of the liquid level within the vessel, the level is determined by calculating the extent to which light is refracted by the surrounding liquid. By definition, these devices require submersion and are therefore susceptible to the problems associated therewith. These devices also suffer from intrinsic safety concerns due to the fact that they are electronic in nature.
Examples of level indicating devices that utilize ultrasonic; microwave and radar principles, and that display the aforementioned problems associated with devices of this nature are displayed by the following:
U.S. Pat. No. 5,406,842, issued in the name of Locke;
U.S. Pat. No. 5,438,867, issued in the name of van der Pol; and
U.S. Pat. No. 5,319,973, issued in the name of Crayton et al.
Finally, other devices that demonstrate methods of level detection and indication, do not necessarily relate directly to the present invention, but warrant inclusion for reference purposes include:
U.S. Pat. No. 4,354,180, issued in the name of Harding;
U.S. Pat. No. 4,670,660, issued in the name of Kuhlen et al; and
U.S. Pat. No. 5,478,966, issued in the name of Sugi.
A search of the previous art did not disclose any patents that read directly on the claims of the instant invention. Consequently, a need has been felt for providing a level indicating device for use in liquid storage vessels that provides accurate indications while avoiding the previously stated problems associated with other conventional methods.